Measurements of Solar Differential Rotation Using the Century Long Kodaikanal Sunspot Data

TL;DR

This study uses a century of digitized sunspot data from Kodaikanal to measure the Sun's differential rotation profile, revealing size-dependent rotation speeds and consistency across solar activity cycles, validated with MDI data.

Contribution

It introduces an automated correlation-based sunspot tracking method applied to long-term data, providing precise measurements of solar rotation parameters and insights into sunspot size effects.

Findings

Larger sunspots rotate slower than smaller ones.

No variation in rotation rates between solar maxima and minima.

Results agree with previous studies and MDI data comparisons.

Abstract

The rotational profile of the Sun is considered to be one of the key inputs in a solar dynamo model. Hence, precise and long-term measurements of this quantity is important for our understanding of solar magnetism and its variability. In this study, we use the newly digitized, white light sunspot data (1923 -- 2011) from Kodaikanal Solar Observatory (KoSO) to derive the solar rotation profile. An automated correlation based sunspot tracking algorithm is implemented to measure the rotation parameters, $A$, the equatorial rotation rate and $B$, the latitudinal gradient. Our measurements of $A=14.381\pm0.004$ and $B=-2.72\pm0.04$ compare well with previous studies. In our analysis, we find that the bigger sunspots (with area $>$400~$\mu$Hem) rotate slower than the smaller ones. At the same time, we do not find any variation in the rotation rates between activity extremes, i.e solar maxima and minima. Lastly, we employ our tracking algorithm on the Michelson Doppler Imager (MDI) data and compare the MDI results with our KoSO values.